1. Field
The present specification generally relates to fritted cover sheets for frit sealing glass substrates and, more specifically, to fritted cover sheets with masks and methods for making the same.
2. Technical Background
U.S. Pat. No. 6,998,776 discloses a method for frit sealing a glass package using a radiation-absorbing glass frit. As generally described in U.S. Pat. No. 6,998,776, a glass frit is deposited in a closed line (typically in the shape of a picture frame) on a first glass substrate and heated to pre-sinter the frit. The first glass substrate is then placed overtop a second glass substrate with the frit disposed between the first and second substrates. A laser beam is subsequently traversed over the frit (typically through one or both of the substrates) to heat and melt the frit, creating a hermetic seal between the substrates.
One use for such a glass package is in the manufacture of organic light emitting diode (OLED) display devices. An exemplary OLED display device comprises a first glass substrate on which is deposited a first electrode material, one or more layers of organic electroluminescent material, and a second electrode material. One characteristic of the organic electroluminescent material is its low damage threshold with respect to heat. That is, the temperature of the electroluminescent material must generally be maintained below about 100° C. to avoid degradation of the material and subsequent failure of the display device. Thus, the sealing operation must be performed in a manner which avoids heating of the electroluminescent material.
A typical scenario for heating the frit to seal OLED display devices includes the use of a laser beam (or other radiant energy source capable of heating the frit to its melting temperature) which is at least as wide as the line of frit deposited on the first substrate. As the frit is heated, care must be taken so as not to inadvertently contact the electroluminescent material with the laser beam. To facilitate heating of the frit while at the same time avoiding undue heating of the electroluminescent material, a mask is sometimes used to ensure the laser beam does not stray from the frit. The mask is placed over the two substrates having the frit sandwiched between them, and the mask and frit are irradiated with the beam. Light from the laser (or other source) which is incident on the mask is either absorbed by the mask or, more preferably, reflected off from the mask as heating of the mask can decrease the useful life of the mask.
As the size of display substrates increase in size the ability to produce masks with the requisite accuracy to prevent inadvertent heating of the electroluminescent material has become challenging. This is particularly important since much of the value of the display is inherent in the deposited electroluminescent materials and other supporting structures (e.g. electrodes) within the device, and error during the frit sealing process has large financial consequences.